Rebooting is not at all drastic, and is typically one of the first things to try. Make sure the Kindle is charged, unplug it, press and hold the power in for a full 30 seconds (time it, and ignore prompts), then release and leave it alone for a few minutes while it restarts.
I have a 10th (latest) gen Kindle I just got like a month ago that is having the same problem. I only have 2 books on it, have only ever used it indoors, and don't leave it charging long once it reaches 100%. It was working totally fine until last week when I was surprised to see it was out of battery. Now I fully charge it to 100% no problem, but it's fully drained by the next day. I make sure to turn the screen off. I've also put it in airplane mode.
I tried what was mentioned above but the "Text in Books" option was grayed out so I assume it was fine. A reboot didn't help.
Anything else to try or do I just have a defective Kindle?
As an update to anyone looking at this historically, a factory reset seems to have fixed the problem for me. Also as a warning, I was often stacking the Kindle vertically with another book I read every morning on my bookshelf. I think setting it right side up, was causing the button to get pressed, which may have also been draining the battery.